1. Field of the Invention
The present invention relates to a method for driving a semiconductor device.
In this specification, a semiconductor device means all types of devices that can function by utilizing semiconductor characteristics, and a semiconductor circuit, a memory device, an imaging device, a display device, an electro-optical device, an electronic device, and the like are all semiconductor devices.
2. Description of the Related Art
Storage devices using semiconductor elements are broadly classified into two categories: a volatile device that loses stored data when power supply stops, and a nonvolatile device that retains stored data even when power is not supplied.
A typical example of a volatile storage device is a dynamic random access memory (DRAM). A DRAM stores data in such a manner that a transistor included in a storage element is selected and electric charge is stored in a capacitor.
On the above-described principle, when data is read from a DRAM, charge in a capacitor is lost; thus, another writing operation is necessary every time data is read out. Moreover, since leakage current (off-state current) flows between a source and a drain of a transistor included in a memory element when the transistor is in an off state, charge flows into or out even if the transistor is not selected, which makes a data holding period short. For that reason, another writing operation (refresh operation) is necessary at predetermined intervals, and it is difficult to sufficiently reduce power consumption. Furthermore, since stored data is lost when power supply stops, an additional storage device using a magnetic material or an optical material is needed in order to hold the data for a long time.
Another example of a volatile storage device is a static random access memory (SRAM). An SRAM holds stored data by using a circuit such as a flip-flop and thus does not need refresh operation. This means that an SRAM has an advantage over a DRAM. However, since a circuit such as a flip-flop is used, an area occupied by a circuit is increased and cost per storage capacity is thus increased. Moreover, as in a DRAM, stored data in an SRAM is lost when power supply stops.
A typical example of a nonvolatile storage device is a flash memory. A flash memory includes a floating gate between a gate electrode and a channel formation region in a transistor and stores data by holding electric charge in the floating gate. Thus, a flash memory has advantages in that the data holding time is extremely long (almost permanent) and refresh operation which is necessary in a volatile storage device is not needed (for example, see Patent Document 1).
However, a gate insulating layer included in a storage element deteriorates by tunneling current generated in writing, so that the storage element stops its function after a predetermined number of writing operations. To reduce adverse effects of this problem, a method in which the number of writing operations for storage elements is equalized is employed, for example. However, a complicated peripheral circuit is needed to realize this method. Moreover, employing such a method does not solve the fundamental problem of lifetime. In other words, a flash memory is not suitable for applications in which data is frequently rewritten.
In addition, high voltage is necessary for injecting charge into the floating gate or removing the charge, and a circuit therefor is required. Further, it takes a relatively long time to inject or remove charge, and it is not easy to increase the speed of writing and erasing data.